Directional radio beam for blind landings



April 25, 1944. l CARL-ERIK GRANQvlsT 2,347,180

DIRETIONAL RADIO BEAM FOR BLIND LANDINGS Filed Feb. 14,' 1942 l. l M

MNEY

Patented Apr. 25, 1944 DIRECTIONAL RADIO BEAM FOR BLIND LANDINGS Carl-Erik Granqvist, Stockholm, Sweden, assignor to Aga-Baltic Aktiebolag, Stockholm. Sweden, a corporation of Sweden Application February 14, 1942, Serial No. 430,903 In Sweden January 13, 1941 (Cl. Z50-11) 3 Claims.

My invention relates to a directional radio beam for blind landing on air-ports.

It has been proposed to arrange dipole antenna systems with a screen, connected to ground, between the two dipole antenna half parts, and to connect alternately one and the other of the two dipole antenna half parts to said screen.

, The screen extends substantially perpendicular to the longitudinal direction of the dipole antennas. The dipole antenna half parts are each connected with one of the two conductors from the radio frequency generator feeding the'transmitter, said generator preferably being of the vacuum tube type.

Ii the reversal of the earth connection between the two dipole antenna half parts is timed so that one of them is connected to earth as the other is disconnected from earth the alternate signals will have different strengths in all directions but one, viz. the direction of the screen. A transmitter of this kind is described in my U. S. patent application Ser. No. 363,281, led October 29, 1940, for Directional radio beacon, now Patent No. 2,309,063, granted January 19, 1943. Transmitters of this kind are particularly suitable for indicating a course in the horizontal direction, but they are not quite suitable for indicating a course in vertical direction, as for instance should be the case for blind landing on an air-port. The present invention relates to a modification of the above mentioned type of beacons, especially suitable for indicating a vertically inclined direction as required for blind landing beacons in air-ports.

According to the present invention impedances are connected between one or both of the dipole antenna half parts and ground and are periodically short-circuited in time with the code which is transmitted by means of the antenna system. The impedances are different. Hence the form of the characteristic curve is changed so that the plane in which a constant sound intensity is heard by a listener no longer coincides with the plane of the screen but is inclined to the screen.

Two embodiments of the invention are shown in the annexed drawing, in which II and I2 indcate the dipole antenna half parts, preferably identical, which are separated by means of the screen I3. The dipole antenna half parts have a length of substantially one quarter of the wave length of the transmitted signal wave.

The arrangements according to Fig. 1 and Fig. 2\ diier in that the screen in the arrangement according to Fig. 1 is less than the critical length, whereas the screen according to Fig. 2 is greater than the critical length one, the critical length of the screen being that at which the radiation diagram from each of the dipole antenna half parts will intersect the plane of the screen substantially perpendicularly with the other dipole antenna half part grounded and without the herein described impedance elements.

With a screen of supercritical length there is obtained from the fed dipole antenna half part, when the other dipole antenna half part is connected to ground, a sacklike characteristic having its major part on the side of the ungrounded antenna half part. If on the other hand the screen has an subcritical length, the sack-like characteristic of the fed antenna part will have its major part on the side of the grounded antenna half part.

In the arrangement according to Fig. l the generator for radio frequency oscillations is indicated at I4. One of its conductors is connected with the lower dipole antenna half part I2 and the other conductor with the screen. The dipole antenna half part Il is connected to the screen by means of a suitable impedance I5, in parallel with a'contact I 1, actuated by a code mechanism I6. The arrangement functions in the following way:

It is rst assumed that the contact Il is closed, so that the dipole antenna half part II is connected to the grounded screen. The screening effect of the screen between the two half parts of the dipole is, however, not complete. Hence a capacitive coupling is present between the two half parts, causing the dipole half part II to radiate with a voltage which is capacitively transferred from the dipole antenna half part I2. Due to the difference in propagation time of the two waves from the respective antennas the radiation from the dipole antenna half part II will be approximately in phase with the radiation from the antenna half part I2 in a direction inclined upwardly from the screen. On the other hand the two radiations in a direction below the screen will be more than displaced in phase, and thus essentially counteract or extinguish each other. The consequence will be that a radiation diagram will be obtained having the form indicated by the curve IB.

When a moment later the contact I1 is opened, the impedance of the dipole antenna half part II to the grounded screen will change, and as a consequence thereof its radiation will have an essentially different character depending upon curve I9.

Now it is obvious that a listener who happens to be higher than the indicated ying direction 20, will hear the signal with stronger intensity when the characteristic has the form I8, and with weaker intensity when the characteristic has the form I9, whereas a listener who happens to be lower than the indicated ilying direction will hear the signal with stronger intensity when it has the characteristic I9, and with weaker intensity when it has the characteristic I8. on the other hand the listener happens to be in the indicated flying line he will hear no diierence in sound intensity, as. both of; the characteristics represent the same signal intensity in this direction.

Preferably the code mechanism I6 is arranged to-close and open the Contact I? in time with a complement code, for instance in time withthe Morse code letters E-T. The pilot may then, by listening to the code, determine if he happens to be higher or lower than the indicated ying direction.

In the arrangement according to Fig. 2 the radio frequency generator is indicated at 2l. Its two conductors are connected with the two dipole antenna half parts Ii and I2 respectively. An impedance 22 is connected between the antenna half part I2 and the screen and is coupled in parallel with a contact 2li actuated by a code mechanism 23. The operation of the arrangement is substantially the same as that of the arrangement according to Fig. l. Vv'hen the dipole antenna half part I2 is directly connected to earth, the radiation only takes place by means of one of the dipole antenna half parts, but in this case it will be the dipole antenna half part I I which directs the radiation, as the screen i3 is of such a length that the capacitive transfer between the two dipole antenna half parts is exceedingly small. The diagram of the radiation is indicated 25. When the contact 24 is opened, in addition to the radiation from the antenna half part Il represented by the diagram Z5, a further radiation will take place from the antenna half part I2, which in certain directions cooperatesv with the radiation 25, but in other directions counteracts this radiation. The radiation diagram for this further radiation will be substantially dependent upon the impedance 22. It will therefore cause a deformation of the diagram 25, which may for instance be assumed to be in the form indicated by the diagram 26.

In this case direction nding takes place in the same manner as with the arrangement according to Fig. l. The course direction. is indicated 21.

Of course the invention is not limited to the embodiments shown in the specification and the drawing, but it may be modified in several manners without departing from the scope thereof. An essential advantage of the arrangements according, to this invention is that they are cirj cular symmetrical, so that the dying angle is the same in diierent azimuth directions. However, if it should be desired to indicate different flying angles in diierent azimuth directions this may easily be accomplished by arranging the antenna system in a manner suitably inclined in` relation to the surfaceV of thev earth.

What is claimed:

1. A radio transmitter for blind landing on air-ports, comprising a dipole antenna having two cooperating dipole antenna half parts and avgrounded screen therebetween extending in a plane normal to said half parts, one of the dipole antenna half parts being connected with the screen through an impedance, a radio frequency generator energizing said antenna to propagate signals therefrom, short circuiting contacts connected in parallel with the said impedance, and a periodically acting code mechanism actuating said contacts to periodically short circuit said impedance in timed sequence for forming code signals.

2. A radio transmitter as set forth in claim 1 in which the screen has a length which is less than the critical length, and the radio frequency generator is connected to the lower dipole antenna and to the screen, said impedance being connected between the upper antenna half part and the screen.

3. A radio transmitter as set forth in claim 1 in which the screen has a length which is greater than the critical length, and a radio frequency generator is connected t0 the dipole antenna half parts, said impedance being connected between the lower dipole antenna half part and the screen.

CARL-ERIK GRANQVIST. 

